I doubt a bad YR would melt the wire.Its for voltage not amps.Amps melt wires not volts unless you try to feed say 110 thru a 22 gauge then it probally wouldnt melt.Fuse depends where the wire melted.At the alt tells you it close to it.At the battery or connections there thats where the problem is.Wires typically burn the most where they short.Thats what cause them to melt.The amps go wild and they cant carry the load.When I wire I run circuit breakers.That way if you do pop one they reset.Fuse or fusiable link you are left in the cold. Rocky